1. Field of the Invention
The present invention relates to a voltage-conversion control method, more particularly to a frequency-hopping control method for a voltage converting circuit that operates in a frequency-hopping mode.
2. Description of the Related Art
Referring to FIG. 1, a conventional DC/DC converting device 900 includes a DC/DC converter 901 and a feedback control circuit 902. The DC/DC converter 901 is a half-bridge LLC oscillating circuit. The feedback control circuit 902 includes an ST L6599 control chip 903.
As the load current decreases (as a result of increasing load resistance Ro), the output voltage Vo of the DC/DC converter 901 increases slightly, causing the voltage at node A, Vcomp, to drop. When Vcomp drops below a preset voltage of the control chip 903, the feedback control circuit 902 stops generating driving signals HVG and LVG, which drive the DC/DC converter 901, such that the DC/DC converter 901 stops voltage conversion operation thereof. During an off-time of the DC/DC converter 901, the output voltage Vo of the DC/DC converter 901 drops slightly, causing Vcomp to rise. Once Vcomp reaches a preset voltage, the feedback control circuit 902 resumes output of the driving signals HVG and LVG, and the DC/DC converter 901 transitions to an on-time.
Therefore, the conventional feedback control circuit 902 controls the operating state of the DC/DC converter 901 according to Vcomp. Further referring to FIG. 2, Vcomp is a gradually-changing voltage. That is to say, during each on-time, duration of Vcomp dropping to the preset voltage of the control chip 903 (see duration marked by t91-t92) is too long in relation to the change in the output voltage Vo. As a result, the DC/DC converter 901 operates for longer than an optimized on-time. As shown by the inductor current Ipri in FIG. 2, the cycles of Ipri during the duration marked by t91-t92 in the on-time convert only a small proportion of the input energy. Therefore, the efficiency of the conventional DC/DC converting device 900, in terms of power loss, deteriorates if the on-time is too long.